lsof – to determine which processes are using a file or directory (useful when trying to figure out what is preventing a device from being umount’d)

netstat – to determine which processes are using network connections (especially useful when trying to figure out which daemon is bound to a certain port) Perl – It is ideal for manipulating text, and since un*x operators live on text files, pipes, input and output, Perl is a great fit.

Cygwin – If you are a un*x admin and have to work on a windows box (even your desktop) having ls, rm, grep, sed, tail etc save you a lot of time when switching OS’s.

sed & awk – Quick examples are command line filtering with sed, and quick and dirty log processing with awk.